Many oil production, transmission, refining, storage, and retail facilities are located adjacent to surface water bodies. Periodic, or sporadic sheens (iridescent films of petroleum hydrocarbons), correlating to low surface water conditions, high surface water conditions, or seasonal conditions, often appear on surface water. Petroleum sheens occur when petroleum liquids (referred to as Nonaqueous Phase Liquids (NAPLs)) with a positive spreading coefficient are introduced to an air-water interface in surface water bodies. NAPLs spread across air-water interfaces until interfacial forces at the leading edge of the sheen are balanced or natural processes deplete the petroleum liquids. Most petroleum NAPLs, including crude oil, fuels, and lubricants, tend to spread across surface water. Spreading can be enhanced by biosurfactants that are associated with biological assimilation of NAPLs in subsurface media including sediments, soils, and rock. Petroleum sheens in surface water can lead to violations of the Clean Water Act and a need for costly remedies.
Sheens are sufficiently thin that the equivalent amount of petroleum liquids can be as low as liters per square kilometer; ten liters per square kilometer equates to a thickness of 0.00001 mm. Clearly, the mass of contaminant that needs to be addressed can be very small when compared to the amounts of petroleum in surface water oil slicks and/or subsurface NAPL releases. Although sheens are commonly associated with releases of petroleum liquids, they can also be due to biological degradation of naturally occurring organics (i.e., plant material). Iridescent coloration of sheens is due to the refraction of light through NAPL layers having varied thicknesses.
Hydrocarbons associated with petroleum are subject to a diverse set of natural attenuation processes. In the case of sheens on surface water, natural loss processes include volatilization, microbially mediated aerobic degradation, and dissolution into water. In sediments and groundwater systems, microbially mediated anaerobic processes can also play an important role in mitigating impacts associated with petroleum at groundwater-surface water interfaces. Depending on the composition of released petroleum and environmental conditions, sheens can persist for minutes to days. Natural losses prevent sheens from being a more common problem.
Mechanisms of releases and factors controlling NAPL assimilation include: seeps, ebullition and erosion. Often, seeps appear at low surface water periods, where groundwater discharges from the banks into surface water, and NAPL near groundwater surface water interfaces, can be driven into surface water. Gases often become entrapped in petroleum impacted sediments or soils through biological processes and/or fluctuating water levels. Similar to surface water, a thin film of petroleum liquid can form between the air and water within a gas bubble in soil. Release of gas bubbles with films of petroleum liquids, from soils or sediment, is referred to as “ebullition.” Often the release of one bubble leads to the coalescing of multiple bubbles and episodic releases. Another mechanism for generating sheens is erosion of sediments and soils. Erosion can occur at high flows along rivers, due to storm-related wave actions, construction activities, and/or ice scour.
Groundwater/surface water interfaces provide transitions between anaerobic and aerobic conditions under which petroleum hydrocarbons are naturally attenuated. Under certain circumstances, including cold weather that constrains rates of biological assimilation, increased upgradient new releases, high water stage, and/or low water stage, as examples, assimilative capacities of bacteria are exceed, and releases occur to surface water in the form of sheens.
FIG. 1A illustrates NAPL, 10, located in ground water, 12, in soil or rock, +seeping, 14, toward ground water/surface water interface, 16, and ultimately forming growing sheen, 18, while FIG. 1B illustrates aerobic attenuation zone, 20, located at soil, 22, water, 24, interface 16, showing seep line, 26. Vertical arrows, 28, illustrate the rise and fall of surface water 24. Note that soil, 22a, is shown as being above the surface water, while soil, 22b, is below the surface water. In FIG. 6, discussed in detail hereinbelow, soil 22a and 22b are shown as being submerged.
Remedies for sheens have included physical barriers (i.e., sheet pile walls), recovery of fluids, and adsorbent barriers. These elements have been employed individually and/or in combinations. For example, remedy elements at a former refinery adjacent to a river included: (a) moving the river; (b) driving a 4,000 foot sheet pile wall through clean fill; (c) maintaining an inward hydraulic gradient across the barrier; (d) hydraulic recovery of upgradient LNAPL; (e) catholic protection of the barrier; (f) temporary sorbent booms; and (g) chronic monitoring. This remedy has been proven to be effective over a fifteen-year period; however, associated costs for capital and operations and maintenance have been large. In some instances, remedies may involve combined technologies. As an example, failure of a capillary barrier can be prevented through hydraulic recovery of NAPL that accumulates upgradient of a capillary barrier. The downside of the combination is that hydraulic recovery is an active, versus a passive, remedy. Many remedies for sheens (even expensive ones) fail. Factors leading to failure include: (a) insufficient adsorption capacity in the barrier for preventing ultimate failure due to overloading; (b) incomplete characterization of impacted soils/sediments; (c) NAPL bypass around or through components; and/or (d) finite life-times of components.